oa Biomarker δ13C Values for Pleistocene Wet/Dry Cycles Record Stable Savanna Vegetation & Variable Alkalinity of Lake Olduvai

Carbon isotope profiles for biomarkers in early Pleistocene lake sediments from Olduvai Gorge document environmental changes associated with alternating wet/dry cycles during a critical episode in hominin evolution. Site 2A of the Olduvai Gorge Coring Project recovered the stratigraphic succession between the Bed I Basalt (∼1.90 Ma) and Tuff IB (∼1.85 Ma) consisting of lacustrine claystones with high Corg contents (avg. 2.5 %). The sequence displays temporal shifts in d13Corg reflecting changes in organic matter (OM) sources. Stratigraphic profiles for d13C values of plant waxes n-alkanes are consistent throughout wet/dry cycles indicating temporal stability in savanna vegetation, refuting evidence for an increase in C4 grasses during drier intervals. d13C values for n-alkanes from aquatic macrophytes increase during drier intervals, whereas higher d13C values for algal biomarkers (sterenes, alkenones) record use of bicarbonate triggered by enhanced lake alkalinity. d13Corg profiles reflect variations in inputs of terrestrial versus aquatic OM during wet/dry cycles, mirrored by hop-17(21)-ene derived from heterotrophic bacteria utilizing sedimentary OM. Thus, d13C records from Paleolake Olduvai sediments show changes in OM sources rather than shifts in the proportion of C3/C4 plants, indicating that intervals of high climate variability do not necessarily cause changes in savanna vegetation affecting hominin habitats.